As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. One of the secondary batteries is a lithium secondary battery having high energy density and discharge voltage, on which much research has been carried out and which is now commercially and widely used.
The secondary battery has attracted considerable attention as a power source for power-driven devices, such as electric bicycles (E-bikes), electric motorcycles, electric vehicles (EV), or hybrid electric vehicles (HEV), as well as a power source for mobile wireless electronic devices, such as mobile phones, digital cameras, personal digital assistants (PDAs), and laptop computers.
A small-sized battery pack having a battery cell packed therein is used for small-sized devices, such as mobile phones and digital cameras. On the other hand, a middle- or large-sized battery pack having battery modules, which includes two or more battery cells connected in parallel and/or in series with each other, packed therein is used for middle- or large-sized devices, such as laptop computers and electric vehicles.
As described above, the lithium secondary battery has excellent electrical properties; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and electrical disconnection, of the lithium secondary battery occur, decomposition of an active material and an electrolyte, which are components of the battery, is caused, whereby heat and gas are generated, and the high-temperature and high-pressure condition caused by the generation of the heat and the gas accelerates the above-mentioned decomposition. Eventually, fire or explosion may occur.
For this reason, the lithium secondary battery is provided with a safety system, such as a protection circuit for interrupting electric current during overcharge, overdischarge, or overcurrent of the battery, a positive temperature coefficient (PTC) element whose resistance is greatly increased so as to interrupt electric current when the temperature of the battery is increased, and a safety vent for interrupting electric current or discharging gas when pressure is increased due to the generation of the gas. In the case of a small-sized cylindrical secondary battery, for example, the PTC element and the safety vent are generally disposed at the upper part of an electrode assembly (a generating element) having a cathode/separator/anode structure, which is mounted in a cylindrical case. In the case of a rectangular or pouch-shaped small-sized secondary battery, on the other hand, the protection circuit module and the PTC element are generally mounted at the upper end of a rectangular case or a pouch-shaped case, in which the generating element is mounted in a sealed state.
The safety problem of the lithium secondary battery is even more serious for a middle- or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells are used in the multi-cell structure battery pack, the abnormal operation of some of the battery cells may cause the abnormal operation of the other battery cells, whereby fire or explosion may occur, which may lead to a large-scale accident. For this reason, the middle- or large-sized battery pack is provided with a safety system, such as a fuse, a bimetal, and a battery management system (BMS), for protecting the battery cells from the overcharge, the overdischarge, and the overcurrent.
However, as the lithium secondary battery is continuously used, i.e., as the battery is continuously charged and discharged, the generating element and the electrically connecting member are gradually degraded. For example, the degradation of the generating element leads to the decomposition of the electrode material and the electrolyte, by which gas is generated. As a result, the battery cell (the cylindrical case or the pouch-shaped case) gradually swells. In the normal state of the lithium secondary battery, the safety system, i.e., the BMS detects the overdischarge, the overcharge, and the overcurrent, and controls/protects the battery pack. In the abnormal state of the lithium secondary battery, however, when the BMS is not operated, a possibility of danger is increased, and it is difficult to control the battery pack for the safety of the battery pack. The middle- or large-sized battery pack has a structure in which a plurality of battery cells is fixedly mounted in a predetermined case. As a result, the respective swelling battery cells are further pressurized in the restrictive case, and therefore, a possibility of fire or explosion is greatly increased under the abnormal operation condition of the battery pack.
Consequently, a technology for fundamentally guaranteeing the safety of the middle- or large-sized battery pack is seriously needed.
In addition, as the size and the weight of the device have been reduced, the demand of a compact-structure battery pack has also increased.